With the ever-increasing data traffic demand in today's mobile networks, immediate solutions for capacity improvement are sought by the operators. Thanks to higher spatial reuse of spectrum, short-radius cells in the range of 50 to 100 meters appear as a promising solution to satisfy bandwidth extensive traffic demands and to enhance the Quality of Experience (QoE) of mobile users.
Heterogeneous Networks (HetNet) are now being deployed, where cells of smaller footprint size (so-called pico, metro or micro cells) are embedded within the coverage area of larger umbrella cells (so-called macro cells), primarily to provide increased capacity in targeted areas of data traffic concentration. HetNet try to exploit the spatial variation in user and traffic distribution to efficiently increase the overall capacity of mobile networks.
Also, with up to 80 percent of the traffic being originated from the indoor where the current mobile networks are least effective due to high buildings penetration losses, indoor data offloading has become a focus of the industry in the recent years. The indoor data offloading incentive is significant firstly due to the successful penetration and the maturity of the fixed broadband technology that can be re-used for mobile network backhauling, and secondly because of the substantial cellular network resources that are expended on penetrating buildings. One attractive solution to offload indoor users' traffic is to deploy femto cells (or home cells). Femto cells are short-range cells operated by subscriber-owned radio access points, and provides improved indoor coverage and increased throughput to home users while off-loading traffic from expensive macro radio access onto the low-cost public Internet.
Seamless handovers between macro and femto cells is considered as one of the major advantages of femto cell technology when compared against other alternative offloading solutions such as Wifi-based solutions. However, there are some issues that need to be addressed to assure smooth and successful users' handover from femto to macro cells.
Generally speaking, there are two types of handovers: hard and soft handovers. In hard handover the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made. For this reason such handovers are also known as break-before-make handovers. On the other hand a soft handover is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence these handovers are called make-before-break handovers.
Before explaining the details and respective issues associated to each handover mode, the uplink power control algorithms used in Wideband Code Division Multiple Access (WCDMA) mobile networks is briefly discussed.
For each activated uplink frequency, the uplink inner-loop power control adjusts the User Equipment (UE) transmit power in order to keep the received uplink Signal to Noise and Interference Ratio (SNIR) on that frequency at a given SNIR target, SNIR_Target. The base station should estimate the SNIR SNIR_Estimate of the received uplink Dedicated Physical Control CHannel (DPCCH). The base station should then generate Transmit Power Control (TPC) commands and transmit the commands once per slot (i.e., once every 0.66 ms) according to the following rule: if SNIR_Estimate>SNIR_Target then the TPC command to transmit is “0”, while if SNIR_Estimate<SNIR_Target then the TPC command to transmit is “1”.
Per 3GPP TS 25.214, there are two algorithms for uplink power control. Each algorithm defines how the TPC commands ought to be interpreted and combined (when received from multiple base stations). In summary, algorithm 2 is more stable compared to algorithm 1 in a sense that it considers five consecutive time slots before making a judgment regarding a change of the transmit power, but is consequently slower than algorithm 1. Also, during the soft handover regime, the UE receives TPC commands from all the cells that it is attached to. However, and regardless of the power control algorithm being used, the TPC combining process is very conservative in a sense that it gives precedence to the base station requiring the lowest uplink transmit power and yielding the least interference.
In hard handover, after the UE establishes its connection to the target cell, it adjusts its transmit power using the open loop power control to estimate the required transmission power for communication with the target cell. The UE's initial transmission consists only of the DPCCH transmission. Per 3GPP TS 25.331 s8.5.3, the UE determines its initial DPCCH transmit power, based on the Received Signal Code Power (RSCP) of the pilot channel (CPICH):DPCCH_Initial_power=DPCCH_Power_offset−CPICH_RSCP  (1)
The test set signals a value for DPCCH_Power_offset that places the UE's initial transmission near the UE target power setting. The UE target power setting is the level that is set for the UE's DPCCH and Dedicated Physical Data CHannel (DPDCH) transmission. Thus, the test set places the initial transmit power of the UE's DPCCH slightly lower than the UE target power setting so that when the DPDCH is turned on, the total UE power matches the UE target power setting.
In classical macro cell deployments, this way of UE's power adjustment is not problematic because firstly the UE's transmit power is not expected to change significantly after the handover as the UE is located at the edges of the two cells far from both antennas (and most likely has been already transmitting on high power to communicate to its previous serving cell). Additionally, as the macro cells normally serve larger number of users (compared to the femto cells), the changes in the transmission power of one single user can not affect the overall uplink interference level of other users noticeably. Unfortunately, this is not the case when considering hard handovers from femto to macro cells operated in the same frequency band (or in overlapping frequency bands). Since macro base station is located at much further distance than the femto base station, the required uplink transmission power to reach the macro base station is significantly higher than the uplink power of the femto cell users. This implies that when the handover is performed, the UE needs to substantially increase its transmit power level so as to communicate with the macro cell. This abrupt and significant change of the transmit power introduces a sudden drop of the SNIR for the other femto cell user(s).
The aforementioned closed-loop power control mechanisms assures that the other femto cell user(s) can sustain the required SNIR at the femto base station despite the changes of the radio channel such as signal fast fades due to users' mobility. However, with the very abrupt and significant drop of the SNIR, it might take long until the appropriate transmission power level is reached. This is especially true when considering multiple users simultaneously increasing their transmission power (which adds to the overall interference level). The case is drastically worst if the aforementioned algorithm 2 is used for transmit power control. Therefore, there is potentially a risk of call drops during the adaptation of the users' transmission power. Even if the call could be maintained, at least severe drop of users' QoE is expected.
Soft handovers may be used too in future femto cells deployment. During the soft handover regime, the UE receives TPC commands from all the cells that it is attached to. However, regardless of the power control algorithm being used, the TPC combining process is very conservative since it is sufficient if the user can at least communicate to one of the base stations. Again this is not a problem for traditional macro cell to macro cell handovers as the required transmission power from the edge of one cell to another cell is not varied substantially. However this is not the case when it comes to soft handovers from femto to macro cells as the user would normally need to transmit with considerably higher power to reach the macro base station. In this case, the TPC command coming from the femto base station during the soft handover regime would keep the user's transmission power low and therefore the user would have difficulty adapting the power quickly enough when he is fully switched to the macro cell (after the handover completes normal power control is used to adapt the transmission power of the user). Again the case is even worse if the aforementioned algorithm 2 is used for transmit power control.